Mycoplasma synoviae is a mycoplasma that is highly infectious to poultry. Mycoplasma synoviae infection is a severe problem in the vast majority of broiler breeder and layer industries. It also often appears in turkey flocks.
Disease caused by Mycoplasma synoviae leads to a decrease in body weight gain and loss of egg production. Morbidity in both chickens and turkeys usually varies between 5-15%. Mortality in chickens is usually low, but may be significant in turkey flocks due to trampling and cannibalism.
Infectious synovitis was first seen at large scale primarily in chickens in growing birds between 4-12 weeks of age in broiler-growing regions in the USA between 1950 and 1960. During that time, the disease was for the first time described and associated with a mycoplasma (Olson et al.; Poult. Sci. 33: 1075 (1954) and Olson et al.; Am. J. Vet. Res. 17: 747-754 (1956)).
Transmission of the infection occurs through the respiratory tract. Natural infection is seen from one week of age in chickens and usually between 10-24 weeks in turkeys. Mycoplasma synoviae infection may occur as a subclinical upper respiratory infection, but can also lead to severe airsacculitis. In other cases, Mycoplasma synoviae becomes systemic and results in infectious synovitis, an acute to chronic infectious disease in chickens and turkeys. This disease is characterised by infection of the synovial membranes of joints and tendon sheaths, producing an exudative synovitis, tenovaginitis or bursitis. Especially problematic is the fact that during vaccination against Newcastle disease or Infectious Bronchitis (or other respiratory poultry pathogens), a standard procedure in almost all chicken-producing countries, animals carrying a M. synoviae-infection are also vaccinated. In these M. synoviae infected animals the ND- and IBV-vaccinations often trigger respiratory infection and air sac infection (Kleven et al.; Avian Dis. 16: 915-924 (1972), Springer et al.; Infect. Immun. 10: 578-589 (1974)). It is clear, that M. synoviae infection, both as a synovial infection and as a respiratory infection, causes great economic damage to the poultry industry. Therefore, efficient vaccines against M. synoviae would be highly appreciated.
So far only inactivated Mycoplasma synoviae vaccines are used in the field. U.S. Pat. No. 3,917,819 discloses an inactivated vaccine against mycoplasma infections. These vaccines however are expensive, since relatively large amounts of antigenic material are necessary to trigger a sufficient immune response. Moreover, all inactivated vaccines have to be manually applied by e.g. eye-drop or parenteral route, requiring individual handling of each individual animal. This is a very labour intensive method of vaccination. Live attenuated vaccines are more desirable, because they have several advantages over inactivated vaccines. First of all, they may comprise less antigenic material because they are self-replicating. Moreover, they give a better protection because they closely mimic the natural infection. As a basis for live Mycoplasma synoviae vaccines, live attenuated Mycoplasma synoviae strains are needed. Only one specific live attenuated strain has been described (Nonomura et al.; Avian Dis. 26: 763-775 (1982)) but no live attenuated vaccine based on this strain has been put on the market. The live attenuated strain by Nonomura, as well as wild-type strains, has the disadvantage, that it must be grown on culture medium containing Nicotinamide Adenine Dinucleotide. This is an expensive component, that can in addition not be sterilised by heating, thus making culture medium preparation more complex. NAD-independence is therefore a highly advantageous feature.
Strains that are adapted to nicotinamide (NIC) instead of NAD are described by DaMassa (DaMassa, A. J. and Adler H. E.; Avian Diseases 19: 544-555, (1975)). These strains however have the disadvantage that they have their original virulence, which makes them not suitable as a basis for live attenuated vaccines.
In addition, the live attenuated strain described by Nonomura, just like the inactivated vaccines, has the disadvantage that it must be administered by dropping into the nostrils of each individual animal. As mentioned before, this is a very labour intensive method of vaccination.
Therefore, a live attenuated strain effective if administered by spraying, and capable of growth on an NAD-free medium is highly desired.